1. Field of the Invention
The present invention relates to a photo mask for use in an exposure process and used for manufacturing semiconductor devices and also relates to a method for manufacturing the same.
2. Description of the Prior Art
A conventional photo mask used for photo-lithography processes, a part of the manufacturing process of semiconductor devices, is so constructed that it is made by forming a certain pattern layer of a thin film layer (a light shielding member) of a metal such as chromium on a glass substrate made of a material such as silica having high tranmittances for an illuminating light of an exposure wavelength. This metal thin film layer is formed on a surface of the substrate by sputtering or evaporation coating, with the thickness enough to almost completely shut off the illuminating light. Further, after a resist layer is laminated, a desired circuit pattern is transferred by a contact or proximity exposure of a master mask or by using a direct-write process, and then an undesired portion of the pattern is removed by a process such as etching, etc. thereby obtaining a finished product.
That is, in the conventional photo mask, the circuit pattern is constructed with a transparent portion having the transmittance of 90% or more and the light shielding portion of almost 0% transmittance.
A photo mask is disclosed for enhancing the contrast of a projected image Japanese Patent laid-open No. 62-50811, for example, discloses a so-called phase-shifting mask in which dielectric thin films, called phase shifters, are deposited on a specified part of the transparent portions to give a phase difference about .pi. radian between light beams passed through the transparent portions adjoining each other. In this phase-shifting mask, the phase of a transmitted light is controlled by the phase shifter, and the phases of the transmited light beams passing through adjoining transparent portions are opposed to each other to enhance the contrast of the image.
However, in the conventional phase-shifting mask, it is necessary to form the phase-shifters selectively at specified positions of the transparent portions after the usual mask processes have been used, with the result that the manufacturing process becomes complex thereby causing defects and thus result in a high cost.
Moreover, in the conventional photo mask, if the pattern formed on the photo mask is coarse in comparison with the resolution of a projecting lens of an exposure apparatus, the distribution of light intensity of the image formed by a projecting lens has a slight increase of intensity near the center of each dark portion (corresponding to the light shielding portion of the photo mask) of the pattern image. Therefore, if the positive resist is used for an object to be projected, the resist on the dark central portion of the pattern image is also exposed, especially when a large exposure amount is applied, and thus, a concave pattern might be formed on the central portion after the resist is developed.
In addition, as for a mask shown in FIG. 16 in which phase shifters 210 are deposited on specified portions of the pattern formed with a light shielding layer 208 on a mask substrate 201, it is often required to control the intensity of each transmission light passing through a portion attached with the phase shifter 210 and another portion without the phase shifter respectively. Especially, in phase-shifting masks of the edge enhancement type (having an auxiliary pattern) and light shielding effect enhancement type, the line width of the opening (a portion deposited with the phase shifter 210) in the light shielding layer 208 should be narrow, as shown in FIG. 17, in order to limit an amount of transmittion light passing through the opening of the light shielding layer 208. However, if the line width of the opening of the shielding layer 208 is too narrow, the opening will not have a predetermined line width or certain separation at the mask process, and therefore it is difficult to obtain a mask suitable for use.